Memory devices are typically provided as internal storage areas for computers. The term “memory” identifies data storage that comes in the form of integrated circuit chips. There are several different types of memory, including RAM (random-access memory). RAM is typically used as main memory in a computer environment. Most RAM is volatile, which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM is lost.
Computers can contain a small amount of read-only memory (ROM) that holds instructions for starting up the computer. An EEPROM (electrically erasable programmable read-only memory) is a special type of non-volatile ROM that can be erased by exposing it to an electrical charge. Like other types of ROM, EEPROM is traditionally not as fast as RAM. EEPROM comprise a large number of memory cells having electrically isolated gates (floating gates). Data is stored in the memory cells in the form of charge on the floating gates. Charge is transported to or removed from the floating gates by programming and erase operations, respectively.
Yet another type of non-volatile memory is a Flash memory. A Flash memory is a type of EEPROM that can be erased and reprogrammed in blocks instead of one byte at a time. Many modem computers have their basic I/O system (BIOS) stored on a flash memory chip so that the BIOS can easily be updated when necessary. Such a BIOS is sometimes called a flash BIOS. Flash memory is also popular in modems because it enables the modem manufacturer to support new protocols as they become standardized.
A typical Flash memory comprises a memory array that includes a large number of memory cells arranged in row and column fashion. Each of the memory cells includes a floating gate field-effect transistor capable of holding a charge. The cells are usually grouped into erasable blocks. Each of the memory cells can be electrically programmed in a random basis by charging its floating gate. The charge can be removed from the floating gate using a block erase operation. The data in a cell is determined by the presence or absence of the charge in the floating gate.
Semiconductor memories, including Flash memory, are commonly built using multi-layering wiring. These memories typically include hierarchical bit lines that are used to retrieve and write data into and from the memory array. The hierarchical bit lines generally include local bit lines and global bit lines. During the manufacture of a semiconductor memory on a wafer, shorts can occur between local bit lines as well as between global bit lines rendering the memory defective.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for efficiently testing a wafer for shorts in both local bit lines and global bit lines.